In fact it did then: former Mexican President Vicente Fox tried to decriminalize small amounts of drugs – marijuana, cocaine, and heroin – for personal use and the thunder from the US effectively killed the proposal. Earlier this year, it passed without as much as a peep. "Wait and see" was the way US officials characterized their stance.

While no one expects any radical shift anytime soon in the US, those who support decriminalization say they see hopeful signs from the Obama administration, especially Washington's silence on recent movements in Latin America, including last month's Mexican law, an Argentina Supreme Court decision calling punishment for small amounts of marijuana unconstitutional, and debates in Ecuador and Brazil which are moving in a similar direction. "If this were the prior administration they would have made hay out of it," says Allen St. Pierre, the executive director of Norml, a nonprofit that argues against marijuana prohibition.

Mr. St. Pierre says that part of the changing view – not shared by all – is a generational evolution that transcends continents and ideologies. The Mexican law was passed under conservative President Felipe Calderón. And a commission this year headed by three former Latin American presidents – from Mexico, Brazil, and Colombia – concluded that the "war on drugs" had failed, as has prohibition. The report calls Europe's approach toward drug use as a matter of public health more humane, while noting that demand there remains a challenge.

"A paradigm shift is required away from repression of drug users and towards treatment and prevention," wrote former Brazilian President Fernando Henrique Cardoso in an editorial after the commission report was released in February.

Health hazards still a concern

But the debate on how to stem drug use and reduce the organized crime that has overtaken swaths of Latin America rages on. In the United Nations 2009 World Drug Report, Antonio Maria Costa, executive director of the United Nations Office on Drugs and Crime, says that enforcement priorities should refocus from drug users to drug traffickers. But he argues forcefully against repealing drug controls on economic, health, and security grounds.

"It does send a mixed message. If this is so bad, how come it's legal in certain economies?" says Roger Noriega, a former assistant secretary of State for Western Hemisphere affairs.

Mexican officials say the intent of decriminalizing small amounts of narcotics is to free up resources and jail space. Yet those proponents of reduced punishments for personal drug use warn that policies may seem more progressive than they actually are. "Everyone has focused on decriminalization in Mexico but the name of the law itself is the Law against Small Drug Traffickers," says Jorge Hernández Tinajero, the president of Cupihd, a civil group in Mexico that disseminates information about drug policies. In other words, he says, its intent is not to make Mexico a new haven for drug users but to direct resources at small-time sellers, hardening Mexico's tough approach toward fighting organized crime.

Though he says he supports the law's distinction between user and criminal, Mr. Hernández Tinajero says that it could end up creating more opportunities for corruption – even though another one of its goals is to reduce it – on the part of the police taking bribes from those carrying more than the legal amounts permitted.

The law goes into effect at a time in Mexico when more than 13,000 people have been killed in drug-related violence in nearly three years. And in the context of violence, which few expect to dissipate any time soon, the new law could provoke a backlash. "It opens up the door to place all the blame on the consumer," he says.

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Part I: Which four countries in Latin America are decriminalizing drugs? Click here to read the story.